Comparison of cohesive powder flowability measured by Schulze Shear Cell, Raining Bed Method, Sevilla Powder Tester and new Ball Indentation Method

Poor powder flow leads to many problems during manufacturing and can lead to inaccurate dosing and offspecificationproducts. Powder flowability is commonly assessed under relatively high applied loads usingshear cells by characterising the unconfined yield strength at a range of applied loads. For applied stressesbelow 1 kPa, it becomes increasingly difficult to obtain reliable values of the unconfined yield strength. Thebulk cohesion and tensile strength of the powder are then obtained by extrapolating the yield locus to zeroand negative loads, respectively. However, the reliability of this approximation for a given material is notknown. To overcome this limitation, techniques such as the Raining Bed Method, Sevilla Powder Tester andthe newly-developed Ball Indentation Method may be used.In this paper, we report our measurement results of the tensile strength of glass beads, α-lactose monohydrateand various sizes of fluid catalytic cracking powders determined by the Sevilla Powder Tester and Raining BedMethod and compare them with those inferred from the Schulze Shear Cell. The results of the latter are also comparedwiththose of the Ball IndentationMethod. The outcome suggests that in the case of shear cell tests, the extrapolationof the yield locus to lower or negative loads is unsafe. The ball indentation method enables thecharacterisation of highly cohesive powders at very low compressive loads; however extrapolation to negativeloads is still not reliable. In contrast, the Sevilla Powder Tester and Raining Bed Methods are able to characterisethe tensile strength directly, but high bulk cohesion poses difficulties as the internal bed failure needs to be analysedin order to reliably estimate the tensile strength. Thesemethods provide a better understanding of powder flow behaviourat low stresses, thus enabling a greater control of manufacturing processes.